1. Field of the Invention
The present invention relates to a method of producing a liquid crystal display device, and more particularly to a liquid crystal display device(hereinafter simply referred to as "LCD") and a fabrication method thereof having a pixel electrode and a counter electrode formed on one substrate device.
2. Description of the Prior Art
Generally, a LCD device of a television or a graphic display includes a pair of transparent glass substrates and a liquid crystal disposed between the glass substrates which include an upper glass substrate and a lower glass substrate. A thin film transistor and a pixel electrode are formed on the lower glass substrate, and a color filter and a counter electrode are formed on the upper glass substrate.
Recently, among LCD devices, a twist nematic mode LCD has been widely used. In the twist nematic mode LCD, there are, however, serious problems regarding the viewing-angle characteristics related to large display area.
Therefore, in order to offer a broad view to the user, an "in plane switching mode" (hereinafter simply referred to as "IPS mode") LCD using a horizontal electric field has been developed. FIG. 1 is a sectional view of a conventional IPS mode LCD.
Referring to FIG. 1, a black matrix 2, is formed on an upper glass substrate 1, and is for improving contrast ratio by preventing light leakage of a thin film transistor formed on a lower glass substrate 5. Color filters 3, for providing the conventional IPS mode LCD with a color display, are then formed at both sides of the black matrix 2. Thereafter, a protective film 4 is formed on the black matrix 2 and the color filters 3.
In order to realize the horizontal electric field on the lower glass substrate 5, a counter electrode 7 and a gate electrode 6A are formed on the lower glass substrate 5. The counter electrode 7 is positioned in line with the gate electrode 6A apart from each other at a predetermined distance. Material used for the gate electrode 6A and the material used for the counter electrode 7 are the same. After forming the gate electrode 6A and the counter electrode 7, a gate insulating film 6B is formed on the gate electrode 6A and the counter electrode 7. A channel layer 6C of amorphous silicon is then formed on a predetermined portion of the gate insulating film 6B at which the gate electrode 6A is formed. Thereafter, a metal film for the wiring of data electrodes is formed on the total structure. A pattern for a source electrode and a pattern for a drain electrode are then formed. At this time, a drain electrode 6D is formed not separately but together with a pixel electrode during the formation step of the pixel electrode, the drain electrode 6D being simply a line extended from a corresponding pixel electrode at a region corresponding to the drain electrode. In addition, the source electrode 6E is formed not separately but together with a data line during the formation step of the data line, the source electrode 6E being only a part of a data line.
A protective film 8 for protecting the thin film transistor and the pixel electrode, and an alignment film are then formed on the total structure. Thereafter, the upper glass substrate 1 and the lower glass substrate 5 are attached to each other. Polarizers 10 and 11 are then respectively attached to the outer surfaces of the upper glass substrate 1 and the lower glass substrate 5. A liquid crystal material (not shown) having a negative dielectric anisotropy(-.DELTA..epsilon.) is inserted and then sealed between the upper glass substrate 1 and the lower glass substrate 5.
If an electric voltage is applied to the IPS mode LCD having the structure as described above, a horizontal electric field is generated. In other words, since the pixel electrode 6D and the counter electrode 7 are positioned on the lower glass substrate 5, a horizontal electric field which is parallel to the plane of the substrate is created. When the electric field is applied to the pixel electrode 6D and the counter electrode 7, LC molecules having negative optical dielectric anisotropy(-.DELTA..epsilon.) are arranged perpendicular to the direction of the electric field. In other words, the molecules are arranged at an angle of 45.degree. from the direction of polarization of an incident light as indicated by the arrow of FIG. 2. Consequently, LCD operates in a bright mode.
FIG. 2 is a plan view of a lower glass substrate illustrated in FIG. 1.
Referring to FIG. 2, a gate line 6A is arranged in a horizontal line. At a portion of the gate line 6A, a data line 6E and the gate line 6A perpendicularly cross each other. A thin film transistor 6 is formed at the crossed portion of the gate line 6A and the data line 6E. The counter electrode 7 has a quadrilateral band shape for defining a unit cell in a space which is created by the data line 6E and the gate line 6A. The pixel electrode 6D has a I-shape with a portion thereof overlapping a portion of the gate line 6A and a portion of the counter electrode 7.
In the conventional IPS mode LCD having the foregoing structure, however, the horizontal electric field has two components: a straight electric field component having electric power lines parallel to the plane of substrate, created between the data line and the counter electrode; and a curved fringe field component F (shown in FIG. 1) having curved electric power lines, created at fringe regions of the data electrode and the counter electrode. The straight electric field component affects a directional orientation of liquid crystal molecules near the lower substrate 1 at an angle of 45.degree., when applying an electric power to the counter electrode and the pixel electrode. The curved fringe field component, however, affects directional orientation of the liquid crystal molecules which are further away from the lower substrate such that these liquid crystal molecules are directionally oriented at an angle of less than 45.degree.. The result is a decrease in transmittance of incident light. In order to compensate for the decrease in light transmittance, the brightness of the back light must be enhanced. Consequently, consumption of electric power is highly increased.